1. Field of the Invention
The present invention relates to a power cable, especially a power cable or power umbilical cable in need of internal cooling.
2. Description of Related Art
The transfer of electrical power through a power cable results in generation of heat within the cable. The present invention relates to transfer of heat caused by the electrical power away from the insulating-, semi conductive- and protective layers surrounding the power conductor(s) in a cable (power cable or power umbilical). A special problem area is power cables arranged inside guide tubes at platforms or other installations above sea level where the guide tube may be filled with air. This section represents a thermal bottleneck and is often the dimensioning factor for the whole cable length. Bend stiffeners are another known thermal bottleneck independently if arranged above or below the sea surface.
To control the temperature of the power cable or power umbilical external or internal cooling may be employed.
U.S. Pat. No. 6,417,457 discloses a single conductor cable for a subsea pipeline heating system. The cable comprises internal grooves or gaps filled with water. The Water enhances cooling especially if circulated through the cable.
U.S. Pat. No. 3,429,979 discloses forced flow of cooling liquid through a hollow conductor and passages within a high voltage cable. The hollow conductor and the passages are connected such that the cooling medium flows in parallel and periodically changes between flowing through the core and through the outer passages. The outer passages can be cooled by the surroundings or a second cooling fluid such as water or air retained by placing the cable within an outer tube. The water may be discharged after having been passed along the cable or it may be recycled or cooled and reused.
U.S. Pat. No. 2,419,053 discloses a cable with a continuous hollow core through which water can be passed for cooling. The cable is adapted to be floating in the sea.
The prior art solutions provide flow passages for cooling water by increasing the cross-sectional diameter of the cable. Either by employing a hollow core, which increases the diameter of the core and thereby the cable, as the cross sectional area of the core material must be equivalent for allowing the same electrical power supply; or by including a passage for cooling water, where the adding of the passage increases the cross sectional area by the cross sectional area of the passage